Studies of ammonia and bicarbonate transport are being conducted in isolated, perfused tubules from rats and rabbits. Studies in rabbit proximal straight tubules showed that 1) ammonia secretion occurs spontaneously by diffusion of NH3 down a concentration gradient generated by luminal acidification; 2) the S-3 portion of the proximal straight tubule generates a spontaneous disequilibrium pH which enhances ammonia secretion; 3) the permeability of the S-2 proximal straight tubule is 2 x 10E-2 cm/s which is adequate to account for ammonia secretion down the measured NH3 concentration gradient; 4) the NH4+ permeability is 5 x 10E-5 cm/s which is consistent with a significant lumen-to-bath backflux of NH4+ in vivo. Experiments in outer medullary collecting ducts from rabbits show 1) the outer stripe portion generates a luminal disequilibrium pH which enhances ammonia secretion by increasing the transepithelial concentration difference driving NH3 diffusion into the lumen; 2) the inner stripe portion does not generate a spontaneous disequilibrium pH despite a rapid rate of proton secretion implying the presence of endogeneous carbonic anhydrase in the apical membrane; 3) both the outer stripe and the inner stripe segments secrete protons at rates that are rapid compared with the cortical collecting duct. Experiments in rabbit cortical collecting ducts show: 1) that this segment lacks endogenous luminal carbonic anhydrase and can generate a spontaneous luminal disequilibrium pH; and 2) that the NH3 permeability is 7 x 10-3 cm/s, a value high enough to account for ammonia secretion by passive diffusion of NH3 down a transepithelial NH3 concentration gradient generated by luminal acidification.